The present invention relates to a color image forming apparatus which forms a latent image on a belt-shaped photoreceptor with an optical writing device using laser beams.
A large number of methods and apparatus have been proposed to form color images by the method of electrophotography.
For example, the following color image forming method is disclosed in the official gazette of Japanese Patent Application Open to Public Inspection No. 100770/1986: Latent images, the number of which corresponds to the number of color separation of a document image, are formed on a photoreceptor drum which functions as an image forming body. Images are transferred onto a transfer drum after each development so that a multicolor image can be formed on the transfer drum. After that, the multicolor image is transferred onto a recording paper and a color copy is obtained.
According to the aforementioned method, it is necessary to provide not only the photoreceptor drum but also the transfer drum, the circumferential surface of which is sufficiently large so that a sheet sized image can be transferred onto the surface. Therefore, it is unavoidable that the dimensions of the apparatus increase and further the structure of the apparatus becomes complicated.
For example, as disclosed in the official gazette of Japanese Patent Application Open to Public Inspection No. 149972/1986, there is a method for obtaining multicolor copies in such a manner that; a latent image is formed on a photoreceptor drum in accordance with the number of separated colors of a document image; and the image is transferred onto a transfer sheet each time when development is conducted. According to the aforementioned method, color images can not be superimposed accurately, so that a color image of high quality can not be provided.
Further, there is a method by which a multicolor image can be obtained in such a manner that; a latent image is repeatedly formed on a photoreceptor drum in accordance with the number of separated colors of a document image; the formed latent image is repeatedly developed with color toner so that color toner images are superimposed on the photoreceptor drum; and the obtained multicolor toner image is transferred onto a transfer paper. The essential process of the aforementioned multicolor image formation is disclosed in the official gazettes of Japanese Patent Application Laid Open Nos. 75850/1985, 76766/1985, 95456/1985, 95458/1985, and 158475/1985 which have been applied by the inventors.
In the case of the aforementioned multicolor image forming apparatus in which color images are obtained through the process of registration, a plurality of developing units accommodating different color toners are disposed around the photoreceptor drum, and in general, when the photoreceptor drum is rotated a plurality of times, latent images on the photoreceptor drum are developed to obtain a multicolor image.
Concerning the image forming body, not only the aforementioned photoreceptor drum, on the circumferential surface of which a photoconductor is coated or vapor-deposited, but also a belt-shaped photoreceptor composed of a flexible belt on which a photoconductor is coated or vapor-deposited, is well known. The shape of a belt-shaped photoreceptor (referred to as a photoreceptor belt, hereinafter) is determined when it is provided around rotating rollers including a drive roller. Therefore, the photoreceptor belt is effective in the case where a compact color image forming apparatus is required, utilizing the space effectively. Further, since a small curvature can be applied to the photoreceptor belt, it is effective to separate a transfer sheet from the surface of the photoreceptor belt in such a manner that; a rotating roller of a small diameter is used to guide the photoreceptor belt; and the transfer sheet is separated from the belt when it passes a position in which the curvature of the photoreceptor belt is small. Due to the foregoing, a failure of separation of a transfer sheet can be prevented.
The inventors have further investigated the technique of utilizing the photoreceptor belt. As a result, the following problems have been found.
(1) The problem will be described as follows: When a multiwriting operation is conducted with an optical writing unit in which laser beams are used as exposure beams to form a multicolor image (in other words, when images of various colors are written on the photoreceptor, being superimposed), since the photoreceptor belt is driven by a roller separate from the belt, a relative positional slippage occurs in the longitudinal direction due to the slippage between the photoreceptor belt and the roller and the wear which is caused in the effective diameter of the roller. According to the aforementioned slippage caused in the moving direction of the photoreceptor belt (that is, in the direction of auxiliary scanning), positional slippage occurs, so that the color slippage in the auxiliary scanning direction and deterioration of resolution are caused.
In order to solve the aforementioned problems, the inventors have already applied for various patents. However, the following problem still remains: In the case where the interval time of scanning conducted by exposure beams does not agree with the time obtained when the period of time required for one revolution of the photoreceptor belt is divided by an integer, slippage within one scanning interval (one scanning pitch) is caused when the position of an image of each color is adjusted in the auxiliary direction.
(2) In order to solve the aforementioned problems, it is necessary to improve the mechanical accuracy of the apparatus. However, improvements in mechanical accuracy are not sufficient to solve the problems. Further, the cost of the apparatus would increase when parts of high accuracy are provided. Therefore, there is a limit to the aforementioned countermeasures.
The present invention has been achieved to solve the aforementioned problems. It is a primary object of the present invention to provide an image forming apparatus characterized in that; when multiwriting is conducted, positional slippage of writing is accurately prevented without severely restricting the position of a photoreceptor in the auxiliary direction; color slippage in the auxiliary direction and deterioration of resolution are prevented; and unnecessary writing is not conducted on the photosensitive portion of the photoreceptor.
Another object of the present invention is to provide a color image forming apparatus which can attain the aforementioned object, avoiding complication of the structure of the apparatus.